Two forms of differentiation are being studied under the simplified conditions of cell culture. The first is adipose conversion of 3T3 cells. This is one of the simplest forms of mammalian differentiation. It is independent of cell-cell interactions but is subject to control by hormones and possibly other factors. During the adipose conversion, a great many enzymes that play a role in triglyceride accumulation become increased in activity, sometimes very strikingly. These enzymes are involved in both lipid synthesis and utilization of external lipid. The system is being examined as a model for fat cell formation in animals. The second system is the growth and terminal differentiation of the epidermal keratinocyte. This is a more complex system, for certain parts require the support of fibroblasts. The mechanism of the terminal differentiation is novel and is being explored at length.